1. Field of the Invention
This invention is concerned with analysis of halide ions in solutions, and in particular with determination of the chloride concentration in acid copper electroplating baths, as a means of providing control over the deposit properties.
2. Description of the Related Art
Electroplating baths typically contain organic additives whose concentrations must be closely controlled in the low parts per million range in order to attain the desired deposit properties and morphology. One of the key functions of such additives is to level the deposit by suppressing the electrodeposition rate at protruding areas in the substrate surface and/or by accelerating the electrodeposition rate in recessed areas. Accelerated deposition may result from mass-transport-limited depletion of a suppressor additive species that is rapidly consumed in the electrodeposition process, or from accumulation of an accelerating species that is consumed with low efficiency. The most sensitive methods available for detecting leveling additives in plating baths involve electrochemical measurement of the metal electrodeposition rate under controlled hydrodynamic conditions, for which the additive concentration in the vicinity of the electrode surface is well-defined.
Cyclic voltammetric stripping (CVS) analysis [D. Tench and C. Ogden, J. Electrochem. Soc. 125, 194 (1978)] is the most widely used bath additive control method and involves cycling the potential of an inert electrode (e.g., Pt) in the plating bath between fixed potential limits so that metal is alternately plated on and stripped from the electrode surface. Such potential cycling is designed to establish a steady-state condition for the electrode surface so that reproducible results are obtained. Accumulation of organic films or other contaminants on the electrode surface can be avoided by periodically cycling the potential of the electrode in the plating solution without organic additives and, if necessary, polishing the electrode using a fine abrasive. Cyclic pulse voltammetric stripping (CPVS), also called cyclic step voltammetric stripping (CSVS), is a variation of the CVS method that employs discrete changes in potential during the analysis to condition the electrode so as to improve the measurement precision [D. Tench and J. White, J. Electrochem. Soc. 132, 831 (1985)]. A rotating disk electrode configuration is typically employed for both CVS and CPVS analysis to provide controlled hydrodynamic conditions.
For CVS and CPVS analyses, the metal deposition rate may be determined from the current or charge passed during metal electrodeposition but it is usually advantageous to measure the charge associated with anodic stripping of the metal from the electrode. A typical CVS/CPVS rate parameter is the stripping peak area (Ar) for a predetermined electrode rotation rate. The CVS method was first applied to control copper pyrophosphate baths (U.S. Pat. No. 4,132,605 to Tench and Ogden) but has since been adapted for control of a variety of other plating systems, including the acid copper sulfate baths that are widely used by the electronics industry [e.g., R. Haak, C. Ogden and D. Tench, Plating Surf. Fin. 68(4), 52 (1981) and Plating Surf. Fin. 69(3), 62 (1982)].
Acid copper sulfate baths are employed in the xe2x80x9cDamascenexe2x80x9d process (e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p.32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.) to electrodeposit copper within fine trenches and vias in dielectric material on semiconductor chips. In the Damascene process, as currently practiced, vias and trenches are etched in the chip""s dielectric material, which is typically silicon dioxide, although materials with lower dielectric constants are under development. A barrier layer, e.g., titanium nitride (TiN), tantalum nitride (TaN) or tungsten nitride (WNx), is deposited on the sidewalls and bottoms of the trenches and vias, typically by reactive sputtering, to prevent Cu migration into the dielectric material and degradation of the device performance. Over the barrier layer, a thin copper seed layer is deposited, typically by sputtering, to provide enhanced conductivity and good adhesion. Copper is then electrodeposited into the trenches and vias. Copper deposited on the outer surface, i.e., outside of the trenches and vias, is removed by chemical mechanical polishing (CMP). A capping or cladding layer (e.g., TiN, TaN or WNx) is applied to the exposed copper circuitry to suppress oxidation and migration of the copper. Alternative barrier/capping layers based on electrolessly deposited cobalt and nickel are currently under investigation [e.g., A. Kohn, M. Eizenberg, Y. Shacham-Diamand and Y. Sverdlov, Mater. Sci. Eng. A302, 18 (2001)]. The xe2x80x9cDual Damascenexe2x80x9d process involves deposition in both trenches and vias at the same time. In this document, the term xe2x80x9cDamascenexe2x80x9d also encompasses the xe2x80x9cDual Damascenexe2x80x9d process.
Acid copper sulfate electroplating baths require a minimum of two types of organic additives to provide good leveling and satisfactory deposit properties. The xe2x80x9csuppressorxe2x80x9d additive (also called the xe2x80x9cpolymerxe2x80x9d, xe2x80x9ccarrierxe2x80x9d, or xe2x80x9cwetterxe2x80x9d, depending on the bath supplier) is typically a polymeric organic species, e.g., high-molecular-weight polyethylene or polypropylene glycol, which adsorbs strongly on the copper cathode surface, in the presence of chloride ion, to form a film that sharply increases the overpotential for copper deposition. The xe2x80x9canti-suppressorxe2x80x9d additive (also called the xe2x80x9cbrightenerxe2x80x9d, xe2x80x9cacceleratorxe2x80x9d or simply the xe2x80x9cadditivexe2x80x9d, depending on the bath supplier) counters the suppressive effect of the suppressor to provide the accelerated deposition needed for good leveling and bottom up filling of Damascene features. From the prior art literature [e.g., J. D. Reid and A. P. David, Plating Surf. Fin. 74(1), 66 (1987); J. J. Kelly, C. Tian and A. C. West, J. Electrochem. Soc. 146(7), 2540 (1999); and R. D. Mikkola and L. Chen, Proc. IEEE 2000 Int. Interconnect Tech. Conf., p. 117 (2000)], the presence of chloride ion is known to be essential to the functioning of the suppressor and anti-suppressor additives in acid copper baths. In order to avoid overplating ultrafine Damascene trenches and vias, a third additive called the xe2x80x9clevelerxe2x80x9d (or xe2x80x9cboosterxe2x80x9d, depending on the bath supplier) is used. The leveler is typically an organic compound containing nitrogen or oxygen that also tends to decrease the copper deposition rate. Plating bath suppliers generally provide additives in the form of solutions that may contain additives of more than one type, as well as other organic and inorganic addition agents. The suppressor additive may be comprised of more than one chemical species and generally involves a range of molecular weights.
In order to obtain satisfactory deposits, the concentrations of the organic additives used in acid copper plating baths must be accurately analyzed and controlled. The suppressor, anti-suppressor and leveler concentrations in acid copper sulfate baths can all be determined by CVS analysis methods based on the effects that these additives exert on the copper electrodeposition rate. At the additive concentrations typically employed, the effect of the suppressor in reducing the copper deposition rate is usually much stronger than that of the leveler so that the concentration of the suppressor can be determined by the usual CVS response curve or dilution titration analysis [W. O. Freitag, C. Ogden, D. Tench and J. White, Plating Surf. Fin. 70(10), 55 (1983)]. Likewise, the anti-suppressor concentration can be determined by the linear approximation technique (LAT) or modified linear approximation technique (MLAT) described by R. Gluzman [Proc. 70th Am. Electroplaters Soc. Tech. Conf., Sur/Fin, Indianapolis, Ind. (June 1983)]. A method for measuring the leveler concentration in the presence of interference from both the suppressor and anti-suppressor is described in U.S. patent application Ser. No. 09/968,202 to Chalyt et al. (filed Oct. 1, 2001).
The concentration of chloride ion in acid copper plating baths must also be closely controlled (typically at a value in the 25 to 100 mg/L range) since chloride is essential to the functioning of the additive system. However, chloride ion specific electrodes are not suitable for use in acid copper plating baths because of the presence of interfering species (e.g., organic additives, copper ions and strong acid) that cause the electrode potential to drift with time. Another prior art method for chloride analysis involves titration with a solution of mercuric nitrate, which is a hazardous material that requires special handling and waste disposal. The calorimetric endpoint for this titration is also difficult to detect with sufficient accuracy, especially for an automated analysis system.
An alternative prior art method for chloride analysis of acid copper plating baths involves potentiometric titration with silver nitrate solution, for which the endpoint detection is readily automated and no hazardous waste is involved. However, the silver chloride precipitate produced during the titration is difficult to remove, and residues of the precipitate, or of a reducing agent (typically, sodium thiosulfate) used to dissolve it, can interfere with subsequent analyses performed in the same cell. The CVS methods used for analyses of organic additives in acid copper baths are particularly sensitive to interference from chloride and silver ions (derived from dissolution of the silver chloride precipitate) and reducing agents, which can affect the copper electrodeposition rate. Another disadvantage of this prior art method is that the silver nitrate solution is decomposed by ambient light and must be handled in darkened containers and tubing, which interfere with visual inspection of the reagent delivery system. In addition, this titration method is only moderately sensitive to chloride.
A sensitive and robust method for analysis of chloride in acid copper plating baths, without the use of contaminating or hazardous chemicals, would be useful for controlling industrial plating processes, particularly those employed by the electronics industry. Such a method would also be useful for other applications, for example, to monitor the quality of the feed water and effluents for industrial processes. A method for detecting other halides is also needed. Chloride ion is generally known to strongly affect the copper electrodeposition rate from acid copper plating baths containing organic additives [e.g., R. D. Mikkola and L. Chen, Proc. IEEE 2000 Int. Interconnect Tech. Conf., p. I1I7 (2000)] but the present inventors were the first to recognize that this effect might be used as a means of quantitative halide analysis.
This invention provides a method for determining the concentration of a halide ion (chloride, iodide or bromide) in an unknown solution from the effect that the halide ion exerts on the copper electrodeposition rate from a copper electrodeposition solution. In this method, a copper electrodeposition rate parameter is measured for the copper electrodeposition solution, a test solution, and a calibration solution. The copper electrodeposition solution includes copper ions, an anion (sulfate, for example), an acid (sulfuric acid, for example), and at least one organic additive at a predetermined concentration. Preferably, the copper electrodeposition solution contains substantially no halide ions, or contains a small predetermined concentration of a halide ion. The test solution comprises the copper electrodeposition solution and a known volume fraction of the unknown solution being analyzed. The calibration solution comprises the copper electrodeposition solution and a known concentration of the halide ion being analyzed, which may be added as a solution or a solid. The concentration of the halide in the unknown solution is determined by comparing the values of the electrodeposition rate parameter measured for the copper electrodeposition solution, the test solution, and the calibration solution. Preferably, a calibration curve is generated by measuring the electrodeposition rate parameter for a plurality of calibration solutions, and the halide concentration in the unknown solution is determined by interpolation of the rate parameter measured for the test solution with respect to the calibration curve. Alternatively, the halide concentration may be determined by the linear approximation method, for which the calibration solution comprises the test solution with a known concentration of the halide added.
The method of the present invention provides a sensitive measure of the halide concentration in the unknown solution since the effect of organic additives on the copper electrodeposition rate is generally small in the absence of halide ion but is large in the presence of halide ion. Consequently, halide derived from addition of the unknown solution to the copper electrodeposition solution (containing little or no halide ion) has a relatively large effect on the copper electrodeposition rate. Addition of halide to the copper electrodeposition solution may increase or decrease the copper electrodeposition rate, depending on the specific organic additives employed in the copper electrodeposition solution.
The method of the present invention is particularly useful for measuring the concentration of chloride ion in an acid copper sulfate electroplating bath. In a preferred embodiment, the copper electrodeposition solution contains the same organic additives as those used in the acid copper plating bath. In this case, addition of chloride ion to the copper electrodeposition solution typically produces a decrease in the copper electrodeposition rate, because of the dominant effect of the suppressor additive. The test solution comprises the copper electrodeposition solution and a known volume fraction of a sample of the copper plating bath. Organic additives present in the plating bath sample are diluted by addition of the plating bath sample to the copper electrodeposition solution so that their effect on the chloride analysis is generally small. A significantly different concentration (including zero concentration) of one or more of the additives may be utilized in the copper electrodeposition solution (compared to the copper plating bath) to improve the sensitivity, selectivity, and/or accuracy of the analysis.
The copper electrodeposition rate is preferably measured by the cyclic voltammetric stripping (CVS) method. A preferred electrodeposition rate parameter is the copper stripping peak area (Ar), which is preferably normalized by dividing the Ar values for the test solution and the calibration solution by the Ar(0) value for the copper electrodeposition solution. The normalized Ar/Ar(0) parameter inherently provides a measure of the difference in the copper electrodeposition rate for a given test or calibration solution relative and that for the copper electrodeposition solution. Use of a normalized rate parameter also minimizes errors resulting from fluctuations in the temperature of the copper electrodeposition solution, and variations in the working electrode surface state. The halide concentration in the test solution is preferably determined by comparison of the Ar/Ar(0) value for the test solution with a calibration plot of Ar/Ar(0) vs. halide concentration (for a plurality of calibration solutions). The halide concentration in the unknown solution may then be calculated from the volume fraction of the unknown solution in the test solution.
The present invention provides a sensitive method for determining the concentration of chloride ions in acid copper plating baths without the use of extraneous reagents. Thus, the cross-contamination and waste disposal issues associated with the reagents and reaction products utilized in prior art methods are avoided. In addition, the method may be practiced using CVS instrumentation, which is widely used for analysis of organic additives in acid copper plating baths. This invention is useful for providing the close control of the chloride concentration in acid copper baths needed for optimum additive functioning and acceptable deposit properties. The method may also be used to measure the concentrations of other halides or halide mixtures that could be used in acid copper electroplating baths. The invention also provides a sensitive measure of the halide concentration in a wide variety of solutions, including drinking water and industrial process feed and effluent solutions.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.